The best way I learned to tell the difference is that the upper jaw length on the Dolly does not pass the eye while the Bulls does. The most difinitive way to tell is that the anal fin on the Dolly doesn't have more than 15 rays while the Bulls have many more.
Dr Eric Taylor at the University of British Columbia has used molecular biology as an effective tool to identify the two species. By identifying their genes, they are able to make accurate differentiation between the two species. In his recent co-published studies, he has found that the two species do interbreed and produce fertile hybrids. This makes visual identification even harder because of the intermediate characteristics.
You can't say "no" Dolly Varden on the east side. Haas and McPhail, Can. J. Fish, Aquat. Sci. 1991, included a specimen from the Wenatchee River, as well as bull trout from the Wenatchee River. But it's certainly more likely to find a bull trout than a Dolly in the Columbia basin. FishPirate sent me this paper because I was interested in the authors attempt at separating the "species" on morphometric characteristics, although FishPirate told me their method hasn't held up as more specimens have been examined, and the authors have backed away from defending their method. In this paper, the best single character was the branchiostegal ray number. The majority of the Dolly Varden have a range of 17 to 23 with a few in the 24-25 range, while most of the bull trout have counts of 26-31 with a few in the 23-25 range. The median branchiostegal ray count for Dolly Varden is 22 and for bull trout 27. The anal fin ray number and the ratio of the upper jaw length to standard length were not as good . Anal fin ray number showed a strong median of 12 whereas Dolly Varden had 11. Bull trout have lager upper jaws in proportion to their body lengths, but the authors found the measurement to have more outliers than than the anal fin count or the branchiostegal count.
They made a linear discriminate function from the branchiostegal counts, the anal fin rays, and the upper jaw to standard length ratio.
You're right-I shouldn't have said flat out "no" to dolly varden. I suppose there could be small, relict populations of dollies in the headwater streams, but it is more likely a bull trout. Haas and McPhail addressed the wenatchee char, and concluded that it is likely "an error or a recent headwater crossover." Looks like the source of the fish is the UW museum; mistakes happen. As you noted, the Haas and Mcphail LDF has come under a lot of fire in our neck of the woods as many of the char group right between dollies and bull trout. Personally, I don't put much stock in the field ID of char, even by experienced field biologists. They may be (and are probably) right on most the time, but I would like to see genetic tests conducted.
Anyway, its cool you get to work with native char, especially in those parts of the state where they are struggling to hold on.
I guess Eric was thinking it might be a bull trout smolt. There have been at least one example of these that passed John Day dam last
year and that fish was strikingly smolted. But this fish was not very silver, the picture is a bit misleading. Also the sheer odds of the
only bull trout/dolly ever found below the WIP/AID headgates being anadromous are fairly low.